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My house has electric smoke detectors wired into an ADT alarm system. I'm not using the alarm system but the smoke detectors seem to work anyway. I'd like to have carbon monoxide detectors too.

Can I replace the existing electric detectors with ones that detect carbon monoxide too?
Or do should I just screw a battery powered one onto the ceiling next to the existing smoke detector?

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    To understand why the smoke detectors continue to work, imagine the lawsuit the monitoring company would face if someone died in a house fire just after canceling their monitoring service. – Niall C. Feb 2 '12 at 3:43
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In our area, local codes made at least one CO detectors mandatory in all condos and rental units. I have had to replace dozens of standard smoke detectors with combo units in hard wired/battery back-up systems. Most every manufacturer makes a CO/Smoke unit that will plug in directly to the existing system. A hard wired replacement is far better than a stand alone battery operated unit, as it will alarm all units in the system if there is a problem.

Having the system monitored or not monitored makes absolutely no difference on how the system works inside the house. All monitored systems will work fine in the house even if the monitoring system is off.

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If you want to keep the system working with the alarm system, then you'll need to replace with a compatible combo device. If you no longer care about the alarm system integration, then just rip it out and replace with a combo, which would probably be cheaper.

The advantage of the alarm system integration (even if it's no longer linked to ADT), is that in the event of a fire you'll probably get whole-house alarming instead of just the alarm detecting the fire going off. That's a BIG benefit, and it probably makes replacing the alarm with a compatible combo unit worth the while.

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    It's worth noting that smoke detectors that integrate with residential security systems usually run on 12V or 24V DC supplied by the control panel whereas standalone units run directly from the mains electricity. Be sure to get the right type. – Niall C. Feb 2 '12 at 3:36
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    Also, if your detectors run from voltage off the panel, make darn sure the panel can't go dead in a power outage! Your panel probably has a backup battery - make sure you know it's limits and get it replaced when it's old. – Michael Kohne Nov 4 '12 at 22:59
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Can I replace the existing electric detectors with ones that detect carbon monoxide too?

Just to clarify, they all don't need to be one or the other, you can mix and match.

The advantage of a hardwired system is that any unit detecting any hazard that it is designed to detect alerts all of the units. So you can put (smoke +) CO detectors close to the potential sources of CO, where there will be the highest concentration. The interconnected units will already be providing a "sympathetic" alarm long before the CO concentration in more "remote" locations is sufficient to directly trigger a detector there.

So, for example, if the only potential source of CO in your house is a furnace and water heater in the basement, you can put a combination smoke+CO detector in the basement and smoke-only detectors in the other hardwired locations. If you're worried about the possibility of that single CO detector failing, you can always put a second combination unit as a "backup" in another strategic location, like the bedroom area.

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The problem with a CO detector combined with a smoke detector mounted on the ceiling is that smoke rises and CO is heavier than air. So by the time a ceiling mounted CO detector goes off you've likely been dead for quite a while. CO monitoring needs to be done low, there are several stand alone CO detection units that plug into a standard wall outlet and have on board battery backup.

Companies that sell CO combined with smoke detectors clearly did not research the product they are attempting to detect.

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    CO is actually slightly lighter than air, and since it's a combustion byproduct it may well be warmer than ambient, so more likely to rise. More info – Niall C. Jul 5 '15 at 19:23
  • @NiallC. is right, CO is lighter than air. I'm pretty sure the only reason CO detectors are sometimes placed down low is because the plug-in models need to be at outlet height for power reasons. Maybe you should do some research before calling manufacturers out as incompetent. – Doresoom Jul 7 '15 at 18:36
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    If anyone wants the science: Carbon monoxide has a molar mass of 28.0, which makes it slightly lighter than air, whose average molar mass is 28.8. According to the ideal gas law, CO is therefore less dense than air. – mjohns Jul 8 '15 at 21:32

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